1. Field of the Invention
This invention relates to a method for the manufacture of nitrogen-containing powders used in making durable ceramic compounds, and, in a preferred embodiment, a method producing high-purity gamma-aluminum oxynitride (AlON) powders, which can be used in making durable transparent ceramic components. The resultant powders are also claimed as part of the invention.
2. Description of Prior Art
This invention relates to the manufacture of powders used in making durable ceramic compounds, in particular durable transparent ceramic components. Gamma-aluminum oxynitride (AlON), for example, has attracted much attention recently due to its unique combination of optical, dielectric and mechanical properties. Of particular interest are its potential applications in infrared missile domes, transparent armor systems, supermarket scanner windows, scratchproof watch crystals and lenses, refractories in metallurgical and materials industries, metal vapor lamp envelopes, and high temperature windows to name a few. Other applications will be apparent to those skilled in the art and the preceding list is by no means exhaustive.
In order to make AlON and other nitrogen-containing powders commercially viable, it is important to develop methods to produce high purity and low cost powders that can, for example, be densified to transparency as in the case of AlON. Heretofore, a variety of techniques has been used including the following: solid state reaction, carbothermal nitridation, melting, and chemical vapor disposition. Up until the present invention, the two procedures most commonly used to synthesize AlON and other nitrogen-containing powders were the solid state reaction and carbothermal nitridation methods.
In the specific case of AlON, the solid state reaction route starts with mixing (usually by ball milling) of appropriate mixtures of aluminum nitride and alumina powders. Annealing or calcining the mixtures in nitrogen atmosphere at temperatures usually above 1700° C. for extended times (usually more than four hours) is required for forming AlON powders. In this well-known solid-state reaction method, expensive AlN powders must be used as an initial ingredient. Another inherent limitation is that the Al2O3 reaction with AlN is sluggish even at high temperatures. Thus, the solid-state reaction method is very time and energy consuming and, due to the sluggishness of the reaction, oxidation can be a problem.
AlON powder has also been alternatively synthesized by a carbothermal nitridation process. In this process, alumina powder is generally mixed with carbon (such as carbon black), and this mixture is then heated in a reactor under nitrogen atmosphere at high temperatures, e.g., 1650–1850° C. The reaction usually takes place via two steps. The first step is the reaction between a portion of alumina, carbon and nitrogen to form aluminum nitride and carbon monoxide. The reaction can occur at 1650–1750° C. The newly formed aluminum nitride then reacts with the remaining alumina at temperatures between 1750–1850° C. to form AlON powder.
Problems with this previous method of carbothermal nitridation include the control of the product stoichiometry. Since reactions only occur at high temperatures (approximately 1800 degrees centigrade), evaporative losses from the charge are likely to be high. Further, the reducing conditions prevailing in a graphite furnace may have a significant influence on the final oxygen to nitrogen ratio in the product powder. AlON powders produced this way often also have some free carbon in the final powder. This impurity is detrimental to the mechanical and, especially, the optical properties of AlON.
In addition to the technical difficulties, limited commercial supply and high cost are other significant hindrances to the widespread application of AlON and other nitrogen containing powders as manufactured by previous methods.
Similar carbothermal nitridation processes have also been used for the manufacturing of other nitrogen-containing compounds, such as SiAlON, TiN, BN, ZrN, HfN, and Si3N4 Hence, the manufacturing and applications of these nitrogen-containing compounds also suffers from the similar difficulties and limitations as mentioned above.